This invention relates generally to solid-state radiation imagers and more particularly to improving the spatial resolution of solid-state radiation imagers through back-side irradiation.
A typical solid-state radiation imager such as an x-ray imager uses a pixelated array of photosensitive elements fabricated on a substrate that is at least partially x-ray opaque. A scintillator coupled to the pixelated array of photosensitive elements receives incident x-rays. Some x-ray imagers have an x-ray transparent protective cover disposed over the scintillator to provide protection for the scintillator. In typical operation, incident x-rays pass through the protective cover into the scintillator. The scintillator absorbs the x-rays and generates light photons that emanate isotropically in all directions. The pixelated array of photosensitive elements receives the light photons that propagate through the scintillator. The pixelated array of photosensitive elements generates electrical signals that correspond to the energy level of the incident radiation. Image processing circuitry coupled to the x-ray imager processes the electrical signals to form an image.
The quality of an x-ray image typically depends on both the total number of x-rays absorbed by the imager and its spatial resolution or the extent of blurring that occurs. Generally, the efficiency of x-ray absorption improves as the thickness of the scintillator increases. Increasing the thickness of the scintillator results in better x-ray absorption and less wasted x-rays. A problem that arises as the thickness of the scintillator increases is that blurring of the image occurs. Blurring happens because most absorption of x-rays occurs at the top portion of the scintillator rather than the bottom portion, which increases the distance that the generated light photons have to propagate to reach the pixelated array of photosensitive elements. The distance is problematic because not all light photons will propagate to the pixelated array of photosensitive elements since the light emanates isotropically in all directions. The more distance that light has to travel to reach the pixelated array of photosensitive elements increases the likelihood of it spreading away from the array, which affects the energy level detected by the array and results in blurring. The blurring will improve as the thickness of the scintillator is reduced. Thus, current x-ray imagers must make a trade-off and select a scintillator thickness that balances x-ray absorption versus blurring.
One approach that has been used to address the trade-off of selecting a scintillator thickness that balances x-ray absorption versus blurring is to use a Cesium Iodide (CsI) scintillator material that has been vapor deposited into individual needles. The scintillator needles act as a light guide enabling the light photons to travel down the needles towards the pixelated array of photosensitive elements. Although the scintillator needles are helpful in directing the light photons towards the pixelated array of photosensitive elements, some spreading of the light still does occur, which results in blurring.
Therefore, there is a need for an x-ray imager that can provide improved image quality by addressing the trade-off of selecting a scintillator thickness that balances x-ray absorption versus blurring. In particular, it is desirable to have an x-ray imager that can have a thicker scintillator for improving x-ray absorption and dose efficiency, while maintaining the same blurring. Similarly, it is desirable to have an x-ray imager that can reduce blurring to increase the spatial resolution of images, while maintaining acceptable x-ray absorption levels.